Split gate flash cell with extremely small cell size

ABSTRACT

A dual-gate cell structure with self-aligned gates. A polysilicon spacer forms a second gate (213) separated from a first gate (201), which is also polysilicon, by a dielectric layer (207). A drain region (219) and a source region (221) are formed next to the gates. In one embodiment, the second gate (213) acts as a floating gate in a flash cell. The floating gate may be programmed and erased by the application of appropriate voltage levels to the first gate (201), source (221), and/or drain (219). The self-aligned nature of the second gate (213) to the first gate (201) allows a very small dual-gate cell to be formed.

BACKGROUND OF THE INVENTION

The present invention relates to integrated circuits (“ICs”), and more particularly to a split-gate flash cell, as may be incorporated in a electronically programmable read only memory (EPROM).

Integrated circuits have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Current ICs provide performance and complexity far beyond what was originally imagined. In order to achieve the improvements in complexity and circuit density, i.e., the number of devices capable of being packed onto a given chip area, the size of the smallest device feature, also known as the device “geometry”, has become smaller with each generation of ICs. Currently, devices are being fabricated with features less than a quarter of a micron across.

Increasing circuit density has not only improved the complexity and performance of ICs but has also provided lower cost parts to the consumer. An IC fabrication facility can cost hundreds of millions, or even billions, of dollars. Each fabrication facility will have a certain throughput of wafers, and each wafer will have a certain number of ICs on it. Therefore, by making the individual devices of an IC smaller, more devices may be fabricated on each wafer, thus increasing the output of the fabrication facility.

Making devices smaller is very challenging, as each process used in IC fabrication has a limit. That is to say, a given process typically only works down to a certain feature size, and then either the process or the device layout needs to be changed. An example of such a limit is the ability to align one layer of the device to a preceding layer of the device.

Several photolithographic steps are commonly used in the fabrication sequence of an integrated circuit. Photolithography is a process that uses a “mask” to expose selected portions of the surface of the wafer or substrate to light, which is shined through the clear portions of the mask. The surface of the wafer is typically coated with a photoresist, and after exposure of selected portions of the photoresist to the light, the photoresist is developed, so that a patterned layer of photoresist remains on the surface of the wafer. Then, any one of several processes, such as an etch process or an implantation process, may be performed to create a selected pattern on or in the substrate, after which process the photoresist is typically stripped. In some conventional fabrication processes each layer of photoresist or patterned material is aligned to the layer or layers below it.

FIG. 1 is a simplified cross section of a split-gate flash cell that illustrates how the need to align one layer to another can limit the smallest size of the device. A first gate 10 patterned from a first layer of polysilicon is formed on the field oxide 12 of the wafer 20. A dielectric layer 14 is formed over the first gate and then, a second layer of polysilicon is formed over the wafer and patterned to form a second gate 16. The second gate has a channel region 18 and an overlap region 22. The overlap region 22 leaves an exposed portion 24 of the first gate 10 that is not covered by the second gate 16.

It is important to accurately align the pattern of the second polysilicon layer to the pattern of the first polysilicon layer. For example, if the exposed portion 24 of the first gate 10 is too small, the second gate 16 may completely cover the first gate 10 and cell program efficiency will degrade in some circumstances. For example, if the floating gate is programmed with channel hot electrons, the hot carrier energy will degrade because V_(DS) will be divided between the first and second polysilicon gaps. If the overlap region 22 is too small, the first gate 10 and second gate 16 may not properly electrically couple, and if the channel region 18 is too small, the transistor may leak, or there may be no operating channel region at all. Therefore, when aligning the mask that will define the features in the second polysilicon layer, it is important that the edge 26 of the second gate 16 is accurately placed in relation to the first gate 10.

If the sizes of the first gate and second gate are not large enough to accommodate the variation associated with the alignment process, some yield loss will occur due to misalignment. Thus, the dimensions of the first and second gate are typically large enough to be compatible with conventional photomask alignment processes and to provide acceptable yields. This may result in device structures that are larger than they need to be for proper circuit operation.

Therefore, it is desirable to provide a multi-gate cell structure that does not require multi-layer alignment of the gates.

SUMMARY OF THE INVENTION

The present invention provides a dual-gate device structure with a small cell size. Such a dual-gate device structure may be used in a split-gate flash cell, for example.

In an exemplary embodiment, a second gate structure is formed by depositing polysilicon over and adjacent to a first gate structure. The second gate structure is separated from the first gate structure by a layer of dielectric material. The second gate is self-aligned to the first gate, so that no photolithographic alignment tolerance is required between these two structures. The first gate and second gate are formed on a substrate having a first conductivity type. Drain and source regions of a second conductivity type are formed in the substrate proximate to the first gate and second gate, separated by a channel region. A dielectric layer separates the first gate from the substrate and a second dielectric layer separates the second gate from the substrate, and a channel region may be formed in the substrate below the gates. In a further embodiment, a pocket region with a higher doping concentration of the first doping type is formed under the second gate to inhibit a conductive channel from forming when not desired.

These and other embodiments of the present invention, as well as its advantages and features are described in more detail in conjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross section of a split-gate cell with a first polysilicon layer aligned to a second polysilicon layer;

FIGS. 2A-2G are simplified cross sections of a portion of an IC illustrating a series of process steps in accordance with one embodiment of the present invention; and

FIGS. 3A-3G are simplified cross sections of a portion of an IC illustrating a series of process steps in accordance with an another embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention provides a compact dual-gate structure. Such a structure can be used in a flash memory cell, for example. The second gate is self-aligned to the first gate, which results in a close spacing of the second gate to the first gate that is controlled by the thickness of an intervening dielectric layer. Both the first gate and the second gate are polysilicon. Although the second polysilicon layer is generally formed after the first polysilicon layer, the first and second gates are on approximately the same plane of the structure, or device. No photolithographic alignment tolerance is required between the first and second gates, and therefore the cell size is very small. The close spacing between gates allows the second gate, which is a floating gate in one example, to be programmed or erased with relatively low, i.e. 5-9 V, voltage levels.

It is understood that the term “polysilicon” is used as an example only and includes doped polysilicon, and that the first or second gate may be formed from a variety of materials, including amorphous silicon, recrystallized amorphous silicon, silicon alloys, such as silicides, and other conductive materials, or that a portion of either gate could be one material, with the remainder of the gate being another material or other materials.

FIGS. 2A-2F are simplified cross sections of a portion of an IC 200 after a series of process steps are used to form one embodiment of a device according to the present invention. FIG. 2A is a simplified cross section of a first polysilicon gate 201 formed on a semiconductor wafer 20. In this instance, the semiconductor wafer is a p-type wafer, but could be an n-type wafer in another embodiment, with appropriate changes to other aspects of the device. A gate dielectric layer 203 was formed on the wafer 20 by an oxidation process, but could be formed by other processes, such as a vapor deposition process. The gate dielectric layer 203 is thermally grown silicon oxide and can be grown in the presence of steam, or in the presence of a nitrogen source, such as ammonia. Growing the gate dielectric layer in the presence of a nitrogen source can result in a silicon oxy-nitride layer. It is desirable that the gate dielectric layer be high-quality dielectric so that it withstands the electric fields associated with use. The first gate 201 was formed by depositing a layer of polysilicon over the gate dielectric layer 203 and then patterning the polysilicon. In some embodiments, the gate dielectric layer is not removed from the field 205 of the wafer 20. In other embodiments the polysilicon is partially alloyed with a silicide-forming element, such as platinum.

FIG. 2B is a simplified cross section of the portion of an IC 200 after a second dielectric layer 207 has been formed over the first gate 201, including the sidewalls 209, 211 of the first gate 201 and the field 205 of the wafer 20. The second dielectric layer 207 is silicon oxy-nitride formed by a chemical vapor deposition process, but could be other materials, such as silicon oxide, formed by similar or different processes.

FIG. 2C is a simplified cross section of the portion of an IC 200 after a second layer of polysilicon has been deposited and patterned to form polysilicon spacers 213, 215. The polysilicon spacers 213, 215 are separated from the sidewalls 209, 211 of the first gate 201 by the second dielectric layer 207, and therefore are self-aligned to the first gate, eliminating the need for a photomask alignment tolerance between the first gate and the second gate.

FIG. 2D is a simplified cross section of the portion of an IC with a layer of photoresist 217 over one of the polysilicon spacers 213 and over a portion of the first gate 201. The photoresist 217 has been exposed with a “slop” mask and developed according to the pattern on the mask. A slop mask is a mask that does not require precise alignment to the existing pattern on a wafer. The dielectric layer 207 overlying the first polysilicon layer will serve as an etch barrier in a subsequent silicon etch process to protect the first polysilicon layer when one of the second polysilicon spacers (i.e. 215) is stripped. In addition to the second dielectric layer 207 shown, an additional dielectric layer (not shown) may lie between the second dielectric layer 207 and the first polysilicon layer 201. The additional dielectric layer may be an oxide layer, for example, formed during the polysilicon anneal process or other process and protected by photoresist during the patterning of the first polysilicon layer.

FIG. 2E is a simplified cross section of the portion of an IC after one of the polysilicon spacers has been removed using an etch process. The second polysilicon spacer forms a second gate 213. In one application, the first gate 201 operates as a select gate, or control gate, and the second gate 213 operates as a floating gate. The floating gate is programmed by channel hot electron injection or band-to-band hot electron injection, and is erased by band-to-band hot hole injection.

FIG. 2F is a simplified cross section of the portion of an IC with a drain region 219 that was formed by a self-aligned implantation process. The drain region 219 is self-aligned to the sidewall 211 of the first gate. A source region 221 is also formed by ion implantation, and a pocket 223 is formed in a separate ion implantation step. It is understood that “source” and “drain” are terms used only as an example and for convenience of reference, and are not intended to limit how the device structure may operate. The pocket 223 has the same carrier type as the substrate, in this case p-type, but at a higher concentration. The pocket 223 inhibits an undesired conductive channel from forming under the second gate 213 when the second gate is charged. The pocket 223 is formed by implanting p-type impurities at an angle to the second gate 213 into the substrate 20. Therefore, the p-type impurities form a pocket around source region 221. The doping level of the source region 221 is N+-type, and is generally higher than the doping level of the pocket 223. Thermal treatment after implantation drives some of the source implant 225 under a portion of the second gate, and some of the drain implant 227 under the first gate.

FIG. 2G is a simplified cross section of an alternative embodiment of a portion of an IC with a drain region 219 that was formed by a self-aligned implantation process. The drain region 219 is self-aligned to the sidewall 211 of the first gate. The first gate 201 is made up of a polysilicon region 202 and a polycide region 204. The polysilicon region 202 is formed by depositing amorphous silicon, and then heating the amorphous silicon to form polycrystalline silicon, or by depositing a polysilicon material. The polycide region 204 is formed by depositing a layer of titanium over the polysilicon and heating the first gate region to form titanium silicide.

A source region 221 is also formed by ion implantation, and a pocket 223 is formed in a separate ion implantation step. It is understood that “source” and “drain” are terms used only as an example and for convenience of reference, and are not intended to limit how the device structure may operate. The pocket 223 has the same carrier type as the substrate, in this case p-type, but at a higher concentration. The pocket 223 inhibits an undesired conductive channel from forming under the second gate 213 when the second gate is charged. The pocket 223 is formed by implanting p-type impurities at an angle to the second gate 213 into the substrate 20. Therefore, the p-type impurities form a pocket around source region 221. The doping level of the source region 221 is N+-type, and is generally higher than the doping level of the pocket 223. Thermal treatment after implantation drives some of the source implant 225 under a portion of the second gate, and some of the drain implant 227 under the first gate.

FIGS. 3A-3G are simplified cross sections of an alternative fabrication process using a polysilicon-fill method. FIG. 3A shows field oxide 300 grown or deposited on a wafer 20, and patterned to open a trench 302 where the first gate will be formed. A high-quality dielectric layer 304, in this case silicon nitride, is deposited over the field oxide 300, bottom 308, and sidewalls 310, 312 of the trench 302.

FIG. 3B shows a polysilicon layer 306 deposited to fill the trench and covering the field oxide 300. The polysilicon is then removed from the field oxide 300 along with the high-quality dielectric layer, leaving the trench 302 lined with the high-quality dielectric layer 304 and filled with polysilicon 306, as shown in FIG. 3C.

FIG. 3D shows the polysilicon first gate 316 separated from the substrate 20 by the high-quality dielectric layer 304, with the high-quality dielectric layer also covering the sidewalls 320, 322 of the first gate 316 after the field oxide has been stripped. A thin layer of thermal oxide 324 is grown on the substrate, but could be deposited as an alternative. Some oxide may form on the exposed portion of the polysilicon (not shown), but this oxide is easily removed later, if desired.

FIG. 3E shows a second layer that has been deposited and patterned to form spacers 326, 328 separated from the first gate 316 by the high-quality dielectric layer 304. The spacers are formed so that the tops 330, 332 of the spacers are approximately the same height from the surface of the substrate as the top 334 of the first gate. A layer of photoresist 336 is applied and developed to cover one 326 of the polysilicon spacers, leaving the other 328 polysilicon spacer exposed so that it may be removed, as shown in FIG. 3F. A layer of dielectric material 327 optionally covers the exposed top surface of the first gate. This layer may be deposited, or preferably grown during a thermal treatment of the first gate. This layer acts as an etch mask for the first polysilicon layer during subsequent processing to remove one of the polysilicon spacers (i.e. 328). This dielectric layer may be left in place or stripped, according to the desired device configuration.

FIG. 3G shows the multiple gate structure after one of the polysilicon spacers has been removed, leaving the other polysilicon spacer as a second gate 338. The second gate 338 is separated from the first gate 316 by the high-quality dielectric layer 304, and is separated from the substrate 20 by the thin layer of thermal oxide 324. A drain region 340, and a source region 342 are implanted, and a pocket region 344 is also implanted, as discussed above.

Examples of typical operating voltages are given in Table 1, below. Three examples of programming schemes are provided. The descriptions of the physical mechanisms used to program and erase the floating gate are believed to be accurate; however, the actual physical mechanisms may be different or more complicated.

TABLE 1 Action V_(G1) V_(S) V_(D) Mechanism Program 5 V (Vcc) 5 V 0 V Channel hot electron program Program 9 V 0 V 5 V Channel hot electron program Program 9 V 5 V floating Band-to-band hot electron injection Erase −5 V or −9 V 5 V floating Band-to-band hot hole injection Read 5 V (Vcc) 0 V 2 V

While the above is a complete description of specific embodiments of the present invention, various modifications, variations, and alternatives may be employed. For example, the present invention may be applied to other types of wafers, such as silicon-on-insulator wafers, or other types of devices with multiple polysilicon layers formed on approximately the same plane of a device. Other variations will be apparent to persons of skill in the art. These equivalents and alternatives are intended to be included within the scope of the present invention. Therefore, the scope of this invention should not be limited to the embodiments described, and should instead be defined by the following claims. 

What is claimed is:
 1. A method of forming, a non-volatile memory cell comprising: (a) forming a first dielectric layer on a surface of a semiconductor substrate, the semiconductor substrate having a substrate conductivity type; (b) forming a first conductive layer on the first dielectric layer; (c) patterning the first conductive layer and first dielectric layer to form a first gate structure separated from the semiconductor substrate by the first dielectric layer, and to form an exposed portion of the surface of the semiconductor substrate; (d) forming a second dielectric layer on a sidewall of the first gate structure and on the exposed portion of the surface of the semiconductor substrate; (e) forming a second conductive layer on the second dielectric layer; (f) patterning the second conductive layer to form a first spacer and a second spacer, the first spacer and the second spacer being separated from the first gate structure by the second dielectric layer; (g) removing the second spacer; (h) forming a first region in the substrate proximate to an opposite sidewall of the first gate structure and a second region in the substrate proximate to an edge of the first spacer, the first region and the second region having a conductivity type opposite to the substrate conductivity type; and (i) forming a pocket region in the semiconductor substrate, the pocket region being proximate to and substantially underlying the second gate structure and proximate to the second region, the pocket region having the substrate conductivity type.
 2. The method of claim 1 further comprising a step, after the step (b) of forming the first conductive layer, and prior to the step (c) of patterning the first conductive layer, of forming an additional dielectric layer overlying the first conductive layer.
 3. The method of claim 2 wherein the additional dielectric layer is an anti-reflective layer.
 4. The method of claim 3 wherein the anti-reflective layer comprises chromium and oxygen.
 5. A method of forming a non-volatile memory cell comprising: (a) forming a first dielectric layer on a surface of a semiconductor substrate, the semiconductor substrate having a substrate conductivity type; (b) defining a trench in the first dielectric layer; (c) forming a high-quality dielectric layer to line the trench; (d) forming a first conductive layer over the high-quality dielectric layer to substantially fill the trench to form a first gate structure; (e) removing at least a portion of the first dielectric layer and a portion of the first conductive layer other than the first gate structure to expose the high-quality dielectric layer on a sidewall of the first gate structure; (f) forming a second dielectric layer over the surface of the semiconductor substrate proximate to the sidewall of the first gate structure; (g) forming a second conductive layer over at least the second dielectric layer; (h) patterning the second conductive layer to form a first spacer separated from the sidewall of the first gate structure by the high-quality dielectric layer and a second spacer on an opposite sidewall of the first gate structure; (i) removing the second spacer; and (j) forming a first region in the substrate proximate to the opposite sidewall of the first gate structure, and a second region in the substrate proximate to an edge of the first spacer, the first region and the second region having a conductivity type opposite to the substrate conductivity type and, (k) forming a pocket region in the semiconductor substrate, the pocket region being proximate to and substantially underlying the second gate structure and proximate to the second region, the pocket region having the substrate conductivity type- has been inserted.
 6. The method of claim 5 wherein the step (f) further comprises forming the second dielectric layer over an exposed portion of the first gate.
 7. A method of forming a non-volatile memory cell comprising: (a) forming a first dielectric layer on a surface of a semiconductor substrate, the semiconductor substrate having a substrate conductivity type; (b) forming a first conductive layer on the first dielectric layer; (c) forming an additional dielectric layer overlying the first conductive layer, wherein said additional dielectric layer is an anti-reflective layer comprising chromium and oxygen; (c) patterning the first conductive layer and first dielectric layer to form a first gate structure separated from the semiconductor substrate by the first dielectric layer, and to form an exposed portion of the surface of the semiconductor substrate; (d) forming a second dielectric layer on a sidewall of the first gate structure and on the exposed portion of the surface of the semiconductor substrate; (e) forming a second conductive layer on the second dielectric layer; (f) patterning the second conductive layer to form a first spacer and a second spacer, the first spacer and the second spacer being separated from the first gate structure by the second dielectric layer; (g) removing the second spacer; and (h) forming a first region in the substrate proximate to an opposite sidewall of the first gate structure and a second region in the substrate proximate to an edge of the first spacer, the first region and the second region having a conductivity type opposite to the substrate conductivity type.
 8. The method of claim 7 further comprising a step, after the step (f) of patterning the second conductive layer, of forming a pocket region in the semiconductor substrate, the pocket region being proximate to the second gate structure and to the second region, and the pocket region having the substrate conductivity type. 